Maybe take a look at this? https://github.com/filebrowser/filebrowser

Proxmox wouldn’t be a bad option to install on your new server if you want flexibility to run multiple OS/VMs and play around to see what works best for you.

As for backups, I use rotating external USB HDDs. I keep one offsite and swap them out every week or so for what is in my opinion a pretty simple and cost effective 3-2-1. Maybe grab another external HDD like the one you already have? Otherwise, you could look into something like Backblaze.